The present invention relates to facsimile transmission systems and particularly to optical scanning systems having spatial synchronization.
Facsimile systems typically scan input copy at a transmitter to form a transmission signal and scan output copy in a receiver in response to the transmission signal. Prior art systems typically have required a rigorous time synchronization between the transmitting station and the receiving station to insure that the output copy is an accurate reproduction of the input copy. A highly accurate electrical clock establishes a reference frequency in the transmitter and a corresponding clock establishes a reference frequency in the receiving station. Although time synchronization between the transmitter and receiver reference frequencies is readily achieved, synchronization problems still arise because of non-uniformity in the scanning mechanisms in the transmitters and receivers. Such non-uniformities result in errors in the output copy when compared with the input copy.
Improvements have been made in optical scanners for use in scanning systems. Ore such improved scanning system employs a combination of ultraviolet and visible laser beams. Such a system is described in the above cross-referenced application. Notwithstanding the improvements in optical scanning systems, problems of spatial synchronization still remain in facsimile systems. Improvements to overcome the existence of spatial nonuniformity are still needed. Additionally, freedom from reliance on time synchronization is desirable in order to achieve greater flexibility of operation of facsimile systems.
In accordance with the above background of the invention, it is an objective of the present invention to provide an improved facsimile system in which spatial errors between the scanned input and output copy are reduced and in which time synchronization is not required.